I am having an issue with my 2006 XK8. After the car is warmed up and sitting at a stop light idling with AC on, the idle will drop to 500-600 rpm. I have had the car go into restricted mode throwing two codes P0128 and P2135. I recently had to replace the thermostat housing. Not sure this has anything to do with it, but as long as I drive the car without using the AC, I have no problems. The car has 60,000 miles on it. Anybody know how to fix my problem?

 

Can you double check the codes?

X103_P_DTC_OBDII_R1.pdf

 

I cleared the codes and since I have been driving without AC, they have not returned.

 

The normal idle rpm is 500-600 rpm. As long as it is running smoothly, its okay.

 

The car normally idles happily between 6 -7 rpms. The problem: I come to a stop sign or slow to a stop in traffic, ac compressor (?) kicks in, creates a drag, motor kind of gulps as idle slows to around 500 and then recovers to normal. Now if I happen to accelerate just as the car is "gulping," the car may go into restricted limp mode. Hopefully I am able to pull over, turn the car off for 30 secs once, maybe twice, start it up and the car runs fine, until I come to a stop and the issue returns. I've figured out that if I press the accelerator and manually keep the idle up or just turn off the ac while at stop light, the car works perfectly.

Maybe unrelated, but I've noticed a bit of play in the transmission coming to a stop in forward and reverse. It will knock slightly. I've had the car go into restricted mode while turning around. Knock knock gulp and croak.

 

I would encourage you to keep these codes, they are there to help, really.

You could also remove the accessory belt and spin all the accessories/idler/tensioner by hand to check for unusual drag.

You might want to take a look at the fuel trims. They might be high, but not quite high enough for a code (25%). You want them in the low single digits (say, 5%). Anything too high and the engine feels lumpy, especially near idle. In other words, you might have air leaks that need to be addressed.

Fuel pressure is also available over OBDII, should be about 55psi consistently.

Best of luck, keep us posted.

 

Check the large black plastic air duct between the air filter box and the throttle body for splits and holes in its pleated accordion-like section. Check the top, bottom, and sides of that entire pleated area. Those plastic pleats get weak as the years and heat cycles pass by, and they tend to split and crack which then allows unmeasured air into the system which can cause some of the symptoms you are experiencing. You may have to remove that duct in order to flex it and inspect it as carefully as required....

 

Hi moonandstars,

P0128 indicates a problem with the coolant thermostat or coolant temperature sensor (CTS). If your thermostat was correctly installed and is working properly, perhaps your CTS is malfunctioning:





P2135 indicates a problem in the Throttle Position Sensor (TPS) to Engine Control Module (ECM) wiring or the TPS itself:




Cheers,

Don